Technical Abstract:
Picking stones up into the mouth and chewing them has been commonly reported in pigs and also in dogs. It has variously been described as play behavior, redirected foraging behavior or a stereotypic behavior. In pigs, stone chewing is often observed in sows housed on paddocks, and most frequently on paddocks where plant cover has been removed. The major issues surrounding stone chewing involve speculation as to its cause and/or function and potential health problems that it may cause the animal in chewing or swallowing the stones. Horrell & A’Ness (1999) observed wild boar in a semi-natural environment, and commercial pigs in six different environments. They reported that commercial sows housed on arable land and housed indoors in small straw yards with stones available spent 46.5% and 40% of observation time respectively chewing stones, which was 10-20% more than sows in three grass-based paddock treatments. Wild boar spent less than 1% of their time chewing stones in their enriched environment. Dailey & McGlone (1997) reported that sows on pasture chewed grass, sows in crates chewed the metal bars of the crates, and sows on a soil-covered paddock chewed stones to a similar extent. Both sets of authors conclude that stone chewing represents thwarted foraging behaviour and may be a form of stereotypic behavior, as it is most prevalent in relatively barren environments of bare earth paddocks. Horrell & A’Ness (1999) hypothesize that it is a function of boredom or a coping response to stress. In terms of health concerns, Davies et al. (2001) examined teeth and stomach contents of indoor- and outdoor-housed sows and found that tooth damage (30% & 28% respectively) and wear (91% & 88% respectively) was in fact very similar in both populations and thus independent of stone-chewing, which was only available to outdoor-housed sows. Even though nearly 40% of outdoor-housed sows had stones in their stomachs, there was no indication that stone-chewing affected the health of the sows.